Our investigations have been directed towards an understanding of the molecular mechanisms governing the regulation of the unlinked genes governing leucine biosynthesis in Neurospora. We are attempting to identify the DNA sequence to which the leu-3 positive product - alpha-isopropylmalate complex binds and study its relationship to transcription, message processing and translation. We are also investigating the relationship between derepression of the leucine system with regulation of the histidine pathway. Mutants that constitutively produce the leucine enzymes are resistant to aminotriazole, an inhibitor of histidine biosynthesis. We plan to continue investigating the reciprocal control mechanism governing the regulation of the two closely linked genes that specify the cytoplasmic and mitochondrial leucyl-tRNA synthetases.